Semiconductor devices, flat panel displays and the like are produced using a variety of plasma apparatuses for example to form oxide film, provide crystal growth for semiconductor layers, and effect etching, ashing and other processes. One such plasma apparatus is a high frequency plasma apparatus using a slot antenna to supply a processing container with a high frequency electromagnetic field which is used to generate a high density plasma. This high frequency plasma apparatus can constantly generate plasma if a plasma gas provides a relatively low pressure. As such it is susceptible to a variety of applications.
FIG. 7 shows one exemplary configuration of a conventional high frequency plasma apparatus. This figure shows a portion of the configuration, as seen in a longitudinal cross section.
This plasma apparatus includes a cylindrical processing container 111 having an opened top and a bottom. At the bottom of processing container 111 is fixed a substrate platform 122 having a surface on which an object to be processed, i.e., a substrate 121 is placed. Processing container 111 has a sidewall provided with a nozzle 117 for supplying a plasma gas and a bottom provided with an exhaust port 116 for evacuation. The processing container 111 opened top is sealed with a dielectric plate 113 to prevent plasma from externally leaking therefrom.
Above dielectric plate 113 a radial antenna 130 corresponding to a type of slot antenna is arranged. Radial antenna 130 is formed of two parallel round conductor plates 131, 132 together forming a radial waveguide 133, and a conductor ring 134 connecting conductor plates 131, 132 at their respective outer peripheral portions. Radial waveguide 133 has an upper surface, or conductor plate 132, having a center provided with an inlet 135 introducing into radial waveguide 133 an electromagnetic field F supplied from a high frequency generator 144 via a circular polarization converter 142. Furthermore, radial waveguide 133 has a lower surface, or conductor plate 131, provided with a plurality of slots 136 concentrically, as shown in FIG. 8A, to allow electromagnetic field F propagating in radial waveguide 133 to be supplied to processing container 111 through dielectric plate 113. Conductor plate 131 acts as a radiation plane of radial antenna 130. Furthermore, radial antenna 130 and dielectric plate 113 have an outer periphery covered with an annular shielding member 112 to prevent electromagnetic field F from leaking externally.
Electromagnetic field F introduced from high frequency generator 144 into radial antenna 130 propagates from the center toward the periphery of radial waveguide 133 radially and is thus radiated through the plurality of slots 136 little by little. Accordingly, radial waveguide 133 has an internal power density high at the center and decreasing gradually as it approaches the periphery. On the other hand, slot 136 provides a coefficient of radiation gradually increasing for a length of L2 of slot 136 increasing from 0, and it is maximized for L2 having a length of one half of a wavelength λg of electromagnetic field F in radial waveguide 133. Accordingly, for slot 136 having length L2 with an upper limit of λg/2, length 2 is conventionally, gradually increased, as seen from the radiation plane's center O toward the periphery, as shown in FIG. 8B, to allow the radial antenna 130 radiation to be uniform across the radiation plane.
If radial antenna 130 having slot 136 with length L2 thus designed is used to generate plasma, however, the plasma is not distributed uniformly, as shown in FIG. 9, as seen in a plane parallel to the substrate bearing surface of substrate platform 122, and would have a high density in a vicinity of the periphery. If substrate 121 is processed in such uneven plasma, the substrate is more rapidly processed at a region underlying the plasma having a high density, resulting in a variation in the processing of the substrate.